The discussion below is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter. Aspects of the invention relates to an assembly of turret and disconnectable buoy, in which the turret at its lower end and the buoy at its upper end are provided with mating coupling provisions.
In offshore oil production, use is made of so called FPSO's (Floating Production, Storage and Offloading). FPSO's are ships which are permanently or semi-permanently anchored offshore at an oilfield for the purpose of receiving well fluids from the oil reservoir, separating the oil from the water and gas and storing the oil on board for later transfer to another vessel. Generally, the FPSO's are anchored by means of single point mooring systems of the internal or external turret type.
In some areas of the world, weather conditions can deteriorate so badly, such as due to hurricanes, that it is required to enable the FPSO to disconnect from its anchoring system and sail temporarily to safer waters. Likewise, in iceberg infested waters it is sometimes necessary to disconnect the FPSO to avoid damage to the vessel.
In these instances use is made of an disconnectable turret mooring system, which can be of the internal or the external type (in the one case the turret is fitted within the confines of the vessel hull, in the other case the turret is fitted forward of the bow of the vessel), comprising a disconnectable buoy, wherein the turret at its lower end and the buoy at its upper end are provided with mating coupling provisions.
In the design of a disconnectable turret particular care needs to be given to the interface between the lower end of the turret and the upper end of the disconnectable buoy.
Ideally this interface is preloaded in such a way as to avoid that a gap is created between the turret and the buoy under external anchoring and wave load conditions. Since generally the turret and the buoy are cylindrical or conical structures with a large diameter, it is also important to interlock these structures in such a way that both act as a single structure when connected. Another objective is to achieve a good fit between the structures at their interface, without resorting to tight and hence expensive machining tolerances.
The assemblies according to the state of the art provide a buoy with a protruding conical upper end which has to be received in a correspondingly shaped conical lower recess of the turret. These known assemblies suffer from the significant drawback that whereas the conical interface eases fit up of the two structures, the conical recess must be fabricated extremely heavy and sturdy to resist the radial loads which follow from the two parts being connected. Very significant preload is required to assure that the buoy and turret do not separate under external loads arising from anchoring and waves. If such preload cannot be achieved sufficiently, locking means, which connect the disconnectable buoy to the turret, are subject to high cyclic load variations following from these anchoring and wave loads and hence may fail prematurely.